Hair seems to help shrimp move through water
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Having hairy legs may make shrimp better swimmers.
“Can hair help you swim better? Swimmers will say no, but shrimp would say yes,” said Sara Oliveira Santos at Brown University in Rhode Island at a fluid dynamics conference this week. She and her colleagues studied how shrimp and shrimp-inspired robots swim to determine whether it is advantageous for them to be hairy.
Shrimp use a technique called metachronal swimming to easily move through water. They beat their specialised appendages, called swimmerets, in just the right sequence to create a wave that travels from their tails to their heads. This generates drag forces that propel them forward instead of slowing them down.
For human swimmers, hairiness makes swimming harder because it increases drag. But many shrimp have swimmerets covered in thin, hair-like structures called setae – and even smaller filaments called setules – that may use drag to help them speed up instead. To determine whether this hairiness makes metachronal swimming easier, the researchers first attached a leg from a dissected shrimp to a mechanical joint, immersed it in water and filmed how it interacted with a jet of dyed fluid. Their recordings showed that under certain conditions, very little liquid got through the setae and the appendage moved like an efficient paddle.
They also built several scaled-up robotic swimming appendages, some with artificial setae and setules and some without any. The researchers moved these through a fluid filled with tiny reflective particles. Recordings of these experiments showed how vortices formed and moved around each robotic leg.
Based on an analysis of this vortex motion, the team concluded that hairier appendages experienced less joint stress than the fully smooth ones. This means having hairs could make it easier for the animals to swim.
Oliveira Santos presented the work at the annual meeting of the American Physical Society Division of Fluid Dynamics in Washington DC on 19 November.
Now, the team is working to further quantify the advantages conferred by hairy legs, such as measuring how much they increase the shrimps’ speed.
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